let windowHeight=window.innerHeight;
window.addEventListener('scroll',()=>{
    let dynamicImg=document.getElementById('dynamic-img2-wrap');
    let originalHeight=dynamicImg.getBoundingClientRect().top;//dynimg距离顶部的距离
    if(originalHeight<=(windowHeight/2)){
        dynamicImg.style.height=(windowHeight/2)-originalHeight+'px';
        // navigation.style.position=fixed;
    }
    
},true);